Screening is the process of rendering the illusion of continuous-tone pictures on displays that are only capable of producing digital picture elements. In the process of printing images, large gray levels of the input picture have to be simulated by the printing device to reproduce a perfect duplicate of the original image. However, in the printed image the pixel resolution can be limited to that which is perceivable by the eye. Hence by grouping the adjacent pixels it is possible to simulate a continuous tone in the image.
Screening may take place by a threshold method in one of two categories: bi-level threshold screening; and multi-level threshold screening. In bi-level threshold screening the (x,y) coordinates of the input pixel are used to index into a two dimensional m by n matrix. The individual entries in the matrix are gray level thresholds which are compared against the input pixel gray level. A binary value (0 or 1) is output based on the results of the comparison. Multi-level screening indexes into a three dimensional lookup table. This three dimensional lookup table is organized as a two dimensional preference matrix of size M by N. The preference matrix is a repeatable spatial tile in the image space. Each entry of the preference matrix has a number of the tone curve which has to be used for the position of (x,y). The tone curve is the compensation transfer function of the input pixel gray value range to within range of the printing process. The tone-curve transfer function is quantized based on a set of thresholds and stored in the form of lookup tables. The lookup tables each contain 2b entries for an unscreened input pixel of size b-bits. All the 2b entries contain the corresponding screened output pixel of size c-bits. This process provides a manner of translating the large dynamic range of the input image into the smaller dynamic range of the printer by mixing colors within the printer dynamic range.
In printer applications the digital nature of the pixel grid imposes constraints on screen angle and line frequency accuracy. A supercell based method helps in achieving higher accuracy for screen angle and screen frequency. The prior art method for the threshold based supercell screening requires significant amount of memory to store threshold array. For example, in a 4 bit marking system (0–15 output gray levels), the memory required to store threshold arrays for a supercell of size 128 by 128 is 4 M bytes.
The size of the supercell is constrained by the availability of memory space for storage of threshold arrays. Thus compromises the ability to meet the requested screen angle and frequency. This turn affects the output image quality. In addition, it not possible to completely cache such threshold arrays in many system configurations, which affects the performance of screening.